This invention relates to felts for use on papermaking machines, and has to do more particularly with felts for removing water from a paper sheet while the sheet is supported on and carried by the felt.
Various techniques have hitherto been proposed for removing water from a paper sheet, the most common being by the use of a press, wherein the paper sheet and the felt on which it is carried are squeezed between rollers, the felt being formed in such a manner that the water extracted from the paper will pass through the felt for discharge. Desirably, the press felts are woven so as to have relatively large open areas or voids which will enhance their water-conveying capabilities so that the water may be removed from the felt upon passage over a suction box.
It has been proposed to enhance the water-conveying capacity of the felt by providing spaced apart monofilaments on the back or machine side of the fabric, the monofilaments lying in spaced apart relation to define water-conveying channels between. Such monofilaments are formed from a synthetic plastic material bonded to the base fabric either by extruding the monofilaments in hot melt form and applying them directly to the base fabric or by adhering them by an adhesive, or by the use of a solvent to render the filaments tacky and hence capable of bonding to the base fabric. While the use of such plastic monofilaments has enhanced the water carrying capability of the felt, particularly when operated under high speed conditions, the use of such monofilaments has involved a number of difficulties. For example, special equipment and handling techniques are required to extrude the plastic monofilaments in molten condition and apply them to the back surface of the base fabric, and this is also true where the filaments are adhesively secured to the base fabric or are softened by a solvent prior to their application to the fabric. It has also been found that after the plastic monofilaments have been in use, they tend to crack and peel away from the base fabric, and additionally, the monofilaments tend to flatten, the net result being a significant decrease in the effectiveness of the channels to perform their intended water conveying function.
More recently, U.S. Pat. No. 4,119,753 discloses felts having improved water conveying channels formed on their machine sides by means of bulky, rib forming yarns which define channels therebetween, the rib forming yarns being securely interlocked with the base fabric and treated with a resin to render them essentially incompressible. Although these felts are capable of being manufactured utilizing conventional equipment which is readily available in mills which manufacture conventional papermaking felts, the process of resin impregnating the rib forming yarns of these felts is difficult, costly, and time-consuming.